Question

Again I’m having troubles while migrating from Access to MySQL.

The following SQL Statements works fine with Access:

SELECT *
FROM tbl_content 
WHERE contentID IN (
     SELECT contentID
     FROM tbl_tags
     WHERE Bezeichnung IN (
              SELECT Bezeichnung
              FROM tbl_tags t2
              WHERE t2.contentID= " & contentID & ")
     AND contentID <> " & contentID & ")
AND Status = 1
ORDER BY Datum DESC LIMIT 0,5;

In MySQLthe performance is really slow. Any ideas for help?

Answer 1

You have answered your own question in the title. Generally speaking, MySQL does not optimise IN (Subquery) as well as JOINS so you would be better off using JOIN

SELECT  *
FROM    tbl_content t1
        INNER JOIN
        (   SELECT  ContentID
            FROM    tbl_tags t1
                    INNER JOIN
                    (   SELECT  Bezeichnung 
                        FROM    tbl_tags t2 
                        WHERE   t2.contentID = " & contentID & "
                        GROUP BY Bezeichnung
                    ) t2
                        ON t1.Bezeichnung = t2.Bezeichnung
            WHERE   contentID <> " & contentID & "
            GROUP BY ContentID
        ) t2
            ON t2.ContentID = t1.ContentID
WHERE   Status = 1
ORDER BY Datum DESC LIMIT 0,5;

Also check that the relevant columns are indexed for increased performance.

EDIT

Further more I think EXISTS May be more efficient still in MySQL, but EXPLAIN should show more:

SELECT  *
FROM    tbl_content t1
WHERE   EXISTS
        (   SELECT  1
            FROM    tbl_tags t2
            WHERE   EXISTS
                    (   SELECT  1
                        FROM    tbl_tags t3
                        WHERE   t2.contentID = " & contentID & "
                        AND     t2.Bezeichnung = t2.Bezeichnung
                    ) 
            AND     t1.ContentID = t2.ContentID
        ) 
AND     Status = 1
AND     t1.contentID <> " & contentID & "
ORDER BY Datum DESC LIMIT 0,5;